The present invention relates to a downhole packer for use in a well bore. More particularly, the present invention relates to a packer which can be used for downhole testing.
During well completions it is desirable to check the integrity of the production bore and any packers used to isolate portions of the well. A known technique for this is to perform an in-flow or negative test. One or more packers are inserted into the well bore to seal off a portion of the well. Low density fluid is introduced to the work string reducing hydrostatic pressure within the pipe. As a consequence of the drop in hydrostatic pressure, well bore fluid flows through any cracks or irregularities into the bore resulting in an increase in pressure which can be monitored and used to indicate where repairs are necessary.
Typically, a separate trip is required to be made into the well to perform an in-flow or negative pressure test. This is because the conventional packer tools used are set by a relative rotation within the well bore. As many other tools are activated by rotation and indeed as the drill string itself would normally be rotated during this type of operation, it is likely that the packer would prematurely set. This problem has been overcome by the introduction of a weight set packer. Such a weight set packer, referred to as a compression set packer, is disclosed in the Applicant's International Patent Application, publication no. WO/0183938. The packer is set by a sleeve moveable on a body of the packer being set down on a formation in the well bore. Movement of the sleeve compresses one or more packing elements to provide a seal.
This compression set packer is particularly suitable for integrity testing of a liner when a permanent packer, or ‘tieback’ packer, with a Polished Bore Receptacle (PBR) has been used. Once the permanent packer with the PBR has been set, a single trip can be made into the well to operate clean-up tools and perform an in-flow or negative test. The clean-up tools may be operated by relative rotation of the work string in the well-bore and further the work string can be slackened off so that the sleeve of the compression set packer lands out-on the PBR. This sets the compression set packer above the PBR and seals the bore between the packers. An in-flow or negative test can then be performed.
A significant disadvantage of this compression set packer is that of loading on the PBR. When an in-flow test is carried out large pressure differentials are created across the packing element and thus a substantial force is applied to the packer from above. In a compression set packer much of this force is transferred to the PBR. As a result, both the packer element and the PBR are at risk of failure if the load bearing capacity is exceeded. This is a particular problem in deep wells were the differential pressures will be greater. For example, if a packer has an annulus surface area, in use, of 10 square inches and a pressure differential applied across it of 30,000 pounds, this provides a force of up to 250,000 pounds at the compression set packer.
The problem of excessive loading and the additional forces on the liner by the hydraulic test pressure differentials has been considered for a liner top test packer as described in WO 03/067027. This discloses an arrangement where the slips are set below a compression set packer and the packer is set against the slips. The additional loading and forces are all then transferred to the casing in which the packer is set via the slips. Thus the slips prevent loading onto any liner or liner hanger located below the slips.
This packer tool, however, has a number of disadvantages. As with all weight-set tools there is a risk that the tool will set in the wrong location if it meets an obstruction in the well bore. As this tool is set by shearing pins and then engaging slips before the packer elements expand, it is difficult to release the tool for repositioning once it has set. Additionally, as the slips move transversely in response to a longitudinally applied force, under excessive longitudinal loading, which can be experienced at high pressure differentials, the slips can loose grip and thus there is a risk of the full force landing on the liner top.